1. Technical Field
The present invention relates to the generation of plasmas and, more particularly, to a circuit for igniting a dc discharge between a first electrode and a second electrode followed by switching the discharge from the second electrode to one, or sequentially, to more than one additional electrodes whereby improved reliability and stability in the switching of the discharge is obtained.
2. Description of Related Art
Plasmas have many industrial applications such that the efficient generation of plasmas is a subject of considerable interest. One recent example relates to the thinning of integrated circuit wafers by plasma etching. Consumers of electronic products are demanding small, lightweight and high performance devices (cell phones, laptops, palmtops and smartcards are a few examples). The integrated circuits and their packages providing the functionality of such devices must occupy smaller spaces but deliver higher performance than their predecessors. These market forces are leading to the development of high-speed ultra-thin integrated circuit chips occupying less space and having multiple layers of dense interconnects. Thus, a need has emerged and is rapidly growing for wafer thinning equipment, and plasma thinning is a leading candidate for meeting these needs of the chip makers. One example of plasma generating equipment to meet these needs is the work of Siniaguine, (U.S. Pat. No. 5,767,627) in which plasmas at atmospheric pressure are generated and used, avoiding the difficulties of performing the plasma etching in a vacuum.
The common plasma discharge used for wafer thinning is dc. However, to ignite the discharge, high frequency and/or high voltage is typically used. That is, a pulse of high voltage dc may be applied to ignite the discharge, followed by application of much lower dc voltage to maintain a stable discharge. Alternatively, high frequency (typically rf) may be applied for the purpose of igniting the plasma then switching to dc for operation. Although dc discharges are the primary focus of the present invention, both dc and rf may be used for ignition.
The generation (ignition) of a discharge in neutral gas typically requires conditions of high voltage and/or high frequency in comparison with the conditions necessary for maintaining a stable discharge once ignited. Thus, high voltage/ high frequency are typically transiently applied to the gas during plasma ignition. The work of Beasley (U.S. Pat. No. 5,914,571) describes the reduction in ignition voltage occurring with increasing frequency (FIG. 1 therein) and circuitry for application of higher frequencies only until the discharge ignites. Unlike the present invention, this work of Beasley relates to the generation of high frequency discharges for purposes of illumination. DC discharges are described by Kim et. al. (U.S. Pat. No. 5,909,086), Sellers (U.S. Pat. No. 5,717,293), Dorfman et. al. (U.S. Pat. No. 5,296,670).
The present invention relates to a plasma generator of the general type described in the work of Siniaguine noted above. In such generator, a discharge is created by application of high ignition voltage and/or high frequency between an electrode and a first nozzle. The nozzles are electrically conducting and serve the dual function of guiding the plasma gas as a nozzle and acting as electrodes for plasma ignition and generation. Following ignition, the discharge is redirected from the first nozzle to a second nozzle for the purpose of moving the plasma flow from the ignition zone into the zone of application to the workpiece, and for avoiding deleterious plating of material from the electrode to the first nozzle that is typically located in close proximity to the electrode (approximately 1.5 mm in some equipment). In practice, switching of the plasma from the first nozzle to a second nozzle has proven to be a difficult process to control reliably and reproducibly. The present invention is directed to plasma ignition circuitry for improving this performance. Alternative embodiments of the present invention relate to switching the plasma from a first nozzle to a second nozzle then sequentially to additional nozzles downstream in the flow of plasma gas. Yet other embodiments of the present invention relate to the generation of two plasma jets directed so as to intersect, and switching current flow from the plasma-igniting nozzle electrodes to a direct flow of current from one electrode through the two plasma jets to the second electrode.